1. Technical Field
This invention relates to impact printers, and more particularly to a control device for an impact printer in which the temperature of an electromagnet unit is directly or indirectly detected to control the printing speed.
2. Prior Art
Low-speed or middle-speed impact printers have been improved to provide low noise, small size, and low manufacturing. In addition, they have been enclosed in a compact housing.
The printing speeds of these impact printers are not high; however, the printers can print more than 1,000 lines per minute, and therefore they should be regarded as heavy duty printers.
Under heavy duty conditions, a large quantiy of heat is generated by the electromagnet unit in which a number of electromagnets are provided with high concentration, and a part of the heat thus generated is stored in the unit. As a result, the temperature of the electromagnet unit may be greatly increased (over the allowable temperature of the electromagnet unit). Therefore, the electrical and mechanical characteristics of the printing hammers are changed, and especially the change of the flight time may result in a difficulty that the resultant print is low in quality; e.g., characters are missing. The flight time is defined as the time elapsed from the generation of a printing order until the start of an actual typing operation due to the printing hammers. In the case of a dot printer, the flight time is the time elapsed until the printing hammer presses a sheet to a platen.
This difficulty may be eliminated by developing high-temperature resisting printing hammers. However, under the present technical conditions the development of such printing hammers is limited. Therefore, one solution is to employ a high-efficiency cooling mechanism to decrease the temperature of the electromagnet unit. However, the use of the cooling mechanism suffers from the following difficulties: the addition of the cooling mechanism requires the use of expensive components such as a blower and a cooling duct, and many other parts must be provided for the installation of these components. As a result, the printer is unavoidably high in manufacturing cost, it may be rather difficult to miniaturize the printer, and sometimes it is necessary to change the housing.
On the other hand, an impact printer is generally operated with a 50% printing duty per page or less. That is, the impact printer is not often used with a 100% printing duty per page nor it is used continuously for a long period. Therefore, it is not economical to design an impact printer whose standard printing mode is a 100% continuous printing mode.